1. Field of the Invention
The present invention relates to signal processing, and more specifically to a signal converter that amplifies and offsets a signal related to the difference between the levels of two signals.
2. Description of the Prior Art
Computer based systems commonly are employed by automated inspection systems to acquire and process data. Often, the sensors employed by the inspection system produce an analog measurement signal, which is related to a measured parameter, that must be converted to digital signals by an analog to digital converter (referred to hereinafter as an "A/D converter") before the signals can be received and processed by the computer. Known inspection systems commonly use peak detection circuits to detect the peak of the signal to extract information pertaining to the value of the measured parameter. The detected peak is compared to a desired value for the parameter to determine whether the measured parameter falls within an acceptable range. Problems related to both resolution and signal to noise ratio exist in many such known inspection systems.
Usually, both the range of the levels of the analog signals that the converter can process and the resolution of the converter are fixed. A converter having a range of zero to ten volts operating with fourteen bits would provide a fixed system resolution of approximately 0.00061 volts per least significant bit (units per least significant bit will be referred to hereinafter as "units/bit"), which may not be the most desirable, or may be unacceptable, for some applications. For example, inspection systems commonly are used to inspect the inside diameter and the outside diameter (referred to hereinafter as "ID" and "OD", respectively) and the wall thickness of fuel tubes that will be used in nuclear power plants. One such inspection system employs ultrasonic sensors to determine the ID, OD, and wall thickness. Each fuel tube has an ID, OD, and wall thickness of a nominal value, that is, the value that the manufacturing process was intended to produce. The inspection system rejects any tube whose actual OD, ID, or wall thickness varies by more than a predetermined amount from its nominal values. Although the nominal value for the OD and ID of a fuel tube varies with the type of fuel tube undergoing inspection, the inspection must be able to inspect tubes with nominal values of up to one inch (2.54 centimeters). Therefore, the A/D converter must be able to receive signals from ultrasonic sensors that represent from zero to one inch (0.0 to 2.54 centimeters). A converter that is capable of receiving signals of a level from zero to ten volts representing zero to one inch (0.0 to 2.54 centimeters) and that operates on the basis of fourteen bits will provide a resolution of approximately 0.00061 volts per bit or approximately 0.000061 inches per bit (0.000155 centimeters/bit). Therefore, it is possible with such an inspection system to detect deviations from the nominal value of only 6.1.times.10.sup.-5 inches, or 0.061 mils (1.55.times.10.sup.-4 centimeters). Considering the criticality of producing and using fuel tubes whose actual parameters deviate from their nominal values by as little as possible, it would be desirable to at least double, and, preferably, increase by a factor of 100, the resolution of the inspection system.
A further problem is caused by the noise injected into the signal by the system or environment, which may be greater than the desired resolution of the system. The peak detector employed by the system is, of course, very sensitive to any noise present in the signal. In many known inspection systems, the computer, the A/D converter and the peak detector receive the measurement signal from the system sensors through a relatively long cable. Thus, a great deal of noise is injected into the measurement signal by the cable before the signal reaches the peak detector. Since the peak detector is very sensitive to noise, system performance is degraded.
The resolution of the system could be improved, for example, doubled, by amplifying the analog measurement signal received from the ultrasonic sensors prior to applying it to the A/D converter. However, such an approach reduces by one half the signal levels that can be produced by the ultrasonic sensors and processed by the A/D converter, thus unacceptably limiting the dimensional range of the inspection system. Further, depending on where in the system noise is injected into the signal, the signal to noise ratio of the system could be further degraded since any noise present in the signal would be amplified along with the signal.
Accordingly, there is a need for a circuit that will increase the resolution of an inspection system without reducing the range of nominal values that can be inspected by the system, and that increases the signal to noise ratio of the system.